The property of active carbon derived from its microporous structure, are well known, among them, character of molecular sieve is worth notice. However, the diameter of micropores on active carbon prepared by conventional processes is usually greater than 10-20 .ANG., so micropores of this size are too large to separate molecules of small size, for example separation of nitrogen and oxygen gas from air, namely character of molecular sieve is inadequate. Several processes of manufacturing active carbon which is applicable for separation of oxygen and nitrogen gas from air, have been disclosed, wherein micropores are partially packed with fine carbon particles and the size become narrow, thus molecular sieving property is increased. For example, such a process is disclosed in Japanese Patent Publication No. 49-37036, wherein small amounts of prepolymer of phenol resin or furan resin are added to active carbon, then polymerized, carbonized by heating at 400.degree.-1,000.degree. C., thus micropore on active carbon is partially packed with carbonious material and endowed with character of molecular sieving. In Japanese Patent Publication No. 52-18675, coke of which volatile material content is less than 5%, is heated to 600.degree.-900.degree. C. in a furnace, then hydrocarbon vapour, such as benzene, toluene and producer gas, is introduced therein, soot generated by decomposition of hydrocarbon partially packs the micropores of the coke, thus a molecular sieve made of coke is formed. In this process, decomposition of hydrocarbon vapour in the furnace is considered to be an indispensable requirement. In Japanese Patent Application Laid Open No. 49-106982, a method to narrow the size of the micropores of coke is described, wherein an organic compound in vapour or in solution is adsorbed onto the surface of coke, thus, the micropore is partially packed. In Japanese Patent Application Laid Open No. 56-130226, a method to narrow the size of the micropores of carbon of size of more than 0.5 mm, is described wherein a concentration of hydrocarbon in vapour of less than 2%, is adsorbed to the surface of carbon and heated, thus the soot generated by the decomposition of the hydrocarbon adhered to the surface of it and narrowed the micropores. In this treatment, hydrocarbon is necessary to have molecular size of more than 0.5 mm such as methane, ethane and styrene. This method comprises to decompose gaseous hydrocarbon and precipitate its soot in the micropores of active carbon or to adsorb gaseous hydrocarbon in these micropores then decompose in it, consequently narrows their size with soot, thus improves the carbons molecular sieving property. In this process, it is necessary to introduce the gaseous hydrocarbon into the furnace and decompose, precipitate soot on adsorbent, so with this process it is complicated and difficult to obtain high quality molecular sieving carbon stably. At the same time, cost of the product becomes expensive. In Nenryokyokaishi vol. 60, No. 654, page 859-864 (1982), a method to improve the molecular sieving property of Yallourn char is described, wherein, 4-5% of exhaust liquor of sulfite pulp and coal tar pitch are added to Yallourn char then pelletized, heated to 600.degree.-700.degree. C. at the speed of 10.degree. C. min.sup.-1 in nitrogen gas and maintained for 1 hour. In this process, the hardness of the pellet is inadequate because of the exhaust liquor of the sulfite pulp used as the binder, and it is difficult to maintain molecular sieving property of the product constantly.